1. Field of the Invention
The present invention relates to an antenna, and especially relates to a multi-antenna structure with multi-input multi-output (MIMO).
2. Description of the Related Art
The technology is progressing every day. It is a technology trend that two LTE (long term evolution)/WWAN MIMO antennas are arranged in a mobile device in order to improve communication quality and speed. However, the low frequency isolation and correlation of the two antennas are usually not good. This is because the radiation field of the low frequency is almost omnidirectional. Therefore, the transmission rate cannot be improved greatly.
FIG. 1 shows a diagram of a conventional two-antenna structure. A conventional two-antenna structure includes a base plate 10, a first antenna 20, and a second antenna 30. A grounded metal surface 101 is arranged on the base plate 10. The grounded metal surface 101 includes two short sides 102 and two long sides 103. The first antenna 20 is arranged on one of the short sides 102. The second antenna 30 is arranged on the other short side 102. The longitudinal current is larger than the transverse current at low frequency because the current path of the two short sides 102 is shorter than the current path of the two long sides 103. Therefore, the current of the first antenna 20 and the current of the second antenna 30 interferes each other easily. The low frequency isolation is bad.
FIG. 2 shows a curve diagram for the isolation analysis of the conventional two-antenna structure. A second curve 50 includes a high frequency isolation 501 and a low frequency isolation 502. The longitudinal current is larger than the transverse current at low frequency because the current path of the two short sides 102 is shorter than the current path of the two long sides 103. The low frequency isolation between the two antennas (−6 dB at the low frequency isolation 502) is bad. Therefore, the current of the first antenna 20 and the current of the second antenna 30 interferes each other easily. The transmission rate cannot be improved greatly.